1. Field of the Invention
This invention discloses an LED (light emitting diode) device, an LED lamp cover structure containing luminescent material, and the method of making LED lamp cover.
2. Background Art
Each LED device can emit a different color of light, and for producing white light, various colors can be combined. A conventional method for producing white light is to use luminescent materials, for example, phosphor materials that at least partially absorb blue LED-emanated light and emit yellow or greenish yellow light. In conventional phosphor-based white LED package, phosphor material is mixed with silicone encapsulation material and dispended in the cup or coated on the LED chip. These methods of applying phosphor luminescent material results in high light loss due to backwardly propagation of phosphor-emitted light into LED chip. This conventional phosphor-based white LED is suffered at higher absorption loss at light output with low correlated color temperature (CCT) such as neutral and warm white light due to high phosphor concentration that increases light trapping factor and increases backward propagation light, and due to higher backward-emitted light by phosphor materials.
An improving method is to separate the phosphor containing layer from the LED die by using a transparent spacer, such as a silicone, to reduce the chance of the phosphor-emitted and phosphor-scattered light entering or reentering the LED chip or the substrate area around the LED chip. This method is disclosed by Lowery in U.S. Pat. No. 5,959,316 and Noguchi et. al., in U.S. Pat. No. 6,858,456. The phosphor layer disclosed by Lowery and Noguchi is a distance from LED chip and is separated from LED chip by a clear encapsulation material. This method can reduce backwardly propagation light entering the LED chip and being trapped there. However, this method does not effectively block backwardly propagation light reaching high absorptive materials such as LED chip because of continuity of material with approximately same reflective index that allows the phosphor-emitted and phosphor-scattered light freely entering the clear layer below the phosphor layer. In US Pat. No. 2005/0239227, Aanegola et. al. discloses an LED package with an air gap between a blue LED package and phosphor layer coated on an inner surface of a separate structure (discrete phosphor-containing structure). Although the phosphor-containing structure separated from the LED package by an air gap can offer a better blocking of light propagating toward the LED package substrate or cup and into LED chip, the LED package using this concept might have light output lower than an LED package with integrated phosphor layer such as the package disclosed by Lowery in U.S. Pat. No. 5,959,316 if the air gap is not optimized. This is because the LED package with a simple discrete phosphor-containing structure can only prevent a portion of light propagating backwardly in backward direction while the amount of excitation light reaching the discrete phosphor layer is less than the integrated-phosphor layer. The lower amount of blue excitation light alleviates or counterbalances the advantage of light blocking improvement in the LED package with a discrete phosphor-containing structure. With a discrete phosphor-containing layer, there is about 40% of light emitted through a bottom surface, according to literature reports such as by Narendran et. al. in his paper published on Phys. Stat. solidi (a) 202 (6), R60-R62, 2005. It means even with an air gap, there is up to 40% of light emitting toward an LED package. This percentage is higher for light output with a lower correlated color temperature (CCT). Therefore, a simple discrete phosphor-containing layer might not significantly improve light output. A method to further blocking this backward propagation light is required. The LED package disclosed in US Pat. No. 2005/0239227 does not provide a method of blocking this amount of backward propagation light. Moreover, coating phosphor materials on a concave surface as disclosed in Aanegola et. al., US Pat. No. 2005/0239227 might cause non-uniform distribution of phosphor materials because of gravity force that causes coating materials flowing to the center of the phosphor-containing structure.